1. Field of the Invention
Embodiments of the present invention relate to a method of forming a semiconductor memory device. In particular, embodiments of the present invention relate to a method of forming improved wiring of a semiconductor memory device.
2. Description of the Related Art
The recent growth of information and communication technologies, e.g., computer industry, has triggered an increased development of semiconductor memory devices, thereby providing highly integrated semiconductor memory devices with improved speed and storage capacity. An increased integration of semiconductor devices may require a decreased design rule and, accordingly, provide reduced distances between circuit elements and reduced margins, which may result in misaligned elements, thereby reducing reliability and yield of the semiconductor memory device. Accordingly, attempts have been made to form multi-layered wiring structures, i.e., a plurality of vertically interconnected metal wiring layers, in order to overcome manufacturing difficulties due to the decreased design rule.
For example, a conventional multi-layered wiring structure may include two wiring layers with an insulation layer therebetween and a via hole through the insulation layer. However, it may be difficult to form a smooth wiring layer, i.e., a conductive layer exhibiting a relatively low surface roughness, thereby causing formation of a non-uniform photosensitive layer thereon. In other words, when a conductive layer includes a surface with a plurality of vertical irregularities, i.e., minute vertical height variations in a form of crests/indents, the photosensitive layer on the conductive layer may be thicker in regions corresponding to indents between the crests.
Light irradiated through the photosensitive layer toward the conductive layer may not be able to reach the conductive layer through thicker portions of the photosensitive layer, thereby forming defected patterns, i.e., unwanted portions of the photosensitive layer may remain unetched. Further, due to non-uniform thickness of the photosensitive material and patterning thereof, etching of the conductive layer may be non-uniform, thereby forming defective wiring, e.g., undesired profile, bridging between adjacent wirings, and so forth. Accordingly, there exists a need for a method of fabricating a semiconductor memory device having wiring layers with smooth surfaces.